Computed defaults

Computed defaults#

Computed fields derive their values from Jinja2 expressions evaluated after the user has answered all interactive prompts. They never appear in the wizard.

How evaluation works#

After the wizard collects the user's answers, Cookieplone evaluates every field marked "format": "computed" in the order they appear in properties.

Each expression is rendered as a Jinja2 template with the current context—including all fields that have been resolved so far.

{
  "properties": {
    "project_slug": {
      "type": "string",
      "title": "Project slug",
      "default": "my-project"
    },
    "package_name": {
      "type": "string",
      "format": "computed",
      "default": "{{ cookiecutter.project_slug | replace('-', '_') }}"
    },
    "module_path": {
      "type": "string",
      "format": "computed",
      "default": "src/{{ cookiecutter.package_name }}"
    }
  }
}

In this example:

  1. project_slug is answered by the user (value: my-project).

  2. package_name is computed from project_slug (result: my_project).

  3. module_path is computed from package_name (result: src/my_project).

Evaluation order#

Properties are evaluated in the order they appear in the properties object. A computed field can reference only fields that appear earlier in properties.

Referencing a field that appears later produces an empty string or raises a Jinja2 error, because that field has not been resolved yet when the expression is evaluated.

Available context#

Inside a computed expression, the Jinja2 context includes:

  • All user-answered fields resolved so far.

  • All computed fields resolved so far.

  • All built-in Cookieplone filters (see Filters reference).

  • The cookiecutter namespace prefix (for example, cookiecutter.project_slug).

Constant fields#

Fields with "format": "constant" are similar to computed fields but simpler. Their default is a literal string—not a Jinja2 expression—and it never changes regardless of user input.

{
  "schema_version": {
    "type": "string",
    "format": "constant",
    "default": "2.0"
  }
}

Use constant fields to embed fixed metadata (schema versions, generator signatures) into every generated project.

Computed fields in --no-input mode#

When --no-input is set, interactive fields use their schema defaults instead of prompting the user. Computed fields still evaluate their Jinja2 expressions against those defaults. The evaluation order and rules are identical.

Why this design#

Cookieplone inherits the Jinja2-in-defaults mechanism from Cookiecutter. Keeping computed fields in the schema (rather than in hooks) means:

  • The derivation logic is visible and auditable in one place.

  • No Python knowledge is required to add derived fields.

  • The wizard can show users the interactive fields without exposing implementation details.

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